1. Field of the Invention
The present invention relates to a capacitor having a tantalum lower electrode and a method of fabricating the same.
2. Background of the Related Art
Capacitors are found in numerous semiconductor devices. Despite the proliferation of various types of capacitors, large amounts of effort are still expended in attempts to obtain capacitors that have particular electrical characteristics and can also be easily fabricated using modern semiconductor fabrication technologies.
One specific capacitor of this type that has been proposed is a capacitor that uses tantalum to form its lower electrode. For example, U.S. Pat. No. 5,142,438 describes such a capacitor. This lower electrode, through buried contacts, electrically connects to the word and bit lines of a semiconductor DRAM memory. The tantalum layer is thereafter subjected to a rapid thermal processing or furnace heating which creates a tantalum silicide layer at the tantalum layer""s interface with the silicon substrate, and an insulating tantalum pentoxide dielectric layer at the top of the tantalum layer. After depositing a layer of barrier material such as silicon nitride, a polysilicon electrode layer is deposited on the structure and doped.
While the above-described approach allows for the creation of a dielectric layer over a conducting layer without specifically applying a distinct dielectric layer, the applications in which this device can be used are limited due to a number of considerations.
For example, when the capacitor is used in a DRAM as disclosed in the ""438 patent, its charge storage need not be very great. Accordingly, the very thin tantalum pentoxide layer that is formed can be used. Consequently, a correspondingly thin deposited tantalum layer can be used to form the tantalum pentoxide layer. If, however, a thicker tantalum pentoxide layer was needed in order to allow for greater charge storage, a thicker original tantalum layer would be used. This cannot, however, be easily achieved, since thick tantalum layers are known to exhibit stress characteristics that lead to warpage of the wafer on which it is formed.
The present invention is made with the above-mentioned problems of the prior art in mind, and it is an object of the present invention to provide a capacitor which can be fabricated on a semiconductor substrate using semiconductor fabrication techniques to provide high capacitance while consuming reduced chip area.
It is a further object of the invention to provide such a capacitor which has switching characteristics variable according to parameters of the fabrication process.
It is yet another object of the invention to provide such a capacitor which can perform a non-volatile charge storage function.
The above objects are achieved by forming a capacitor in which, according to a first preferred embodiment of the invention, a titanium nitride layer is deposited on a silicon substrate for stress reduction and adherence promotion, and a layer of tantalum is deposited thereon. Then, the tantalum layer is oxidized using a furnace or RTA to produce a tantalum pentoxide layer. A top electrode of metal or polysilicon is then deposited on the tantalum pentoxide layer.
According to one aspect of the invention, the top electrode is made from polysilicon or a similar semiconducting material so that a space charge layer will form in the electrode which will change the rate at which the capacitor charges and discharges. Other fabrication parameters described in greater detail herein also affect the charging characteristics of the resultant capacitor.
According to another aspect of the invention, the top electrode is preferably made from metal to provide an optimal linear response for use in analog applications. An appropriate application for a capacitor designed in this way would be for tuning a receiver circuit or the like.
According to yet another aspect of the present invention, an undoped polysilicon layer above the tantalum pentoxide layer between the top and bottom electrodes stores charge for non-volatile memory applications. For this purpose, polysilicon can be used to form the top electrode; alternatively, materials such as silicon nitride may be used.